Economical Polar-Axis Solar Tracker for a Circular Reflective Dish

ABSTRACT

The most urgent need in the field of solar energy is to lower the final cost per watt of all involved components. Some of the most expensive components within a solar tracker go into the precision drive system which accurately follows the motion of the sun. This invention reduces the complexity and the required number of drive components in a tracker optimized for reflective dishes. With this invention a single drive motor can keep 20 or more large reflective dishes accurately tracking the sun, whereas 40 drive motors with more complex control systems would typically be required for the same number of dishes. In addition this invention allows for complete inversion of the dishes, which helps reduce dust accumulation on the optical surfaces and lowers wind resistance during storms.

PRIORITY CLAIM

This application is a continuation-in-part of Indian patent application No. 2010/DEL/2007 filed on Sep. 24, 2007 and entitled “An Economical Solar Tracker for a Concentrating Reflective Dish”.

FIELD OF THE INVENTION

The invention relates to solar energy, and more specifically to methods for tracking the motion of the sun as it moves through the sky, enabling more efficient concentration and utilization of the energy coming from the sun.

BACKGROUND OF THE INVENTION

In order to accurately track the motion of the sun through the sky, it's necessary to somehow replicate that motion. The majority of solar trackers deal with the sun's motion as if it were an arbitrary series of azimuth-elevation (az-el) coordinates. They generally depend on complex computer software to predict those coordinates, and/or a sophisticated, closed-loop, two-axis control system to move the tracker so that it accurately follows the motion of the sun. Such solar trackers are called az-el trackers, and two examples of them are illustrated in FIGS. 1 a and 1 b. Note that the dish in FIG. 1 a is high above the ground, which is a distinct disadvantage during windstorms. A further disadvantage is that the torques required to properly orient such a tracker are significant, since the full weight of the dish plus the solar energy receiver must be lifted in order to aim the tracker properly. FIG. 1 b shows an az/el tracker which has its dish somewhat lower to the ground. The weight of the solar energy receiver (a Stirling engine, in that case) also balances the weight of the reflector dish, which is an advantage in that greatly reduced torques are required to maneuver the dish as it tracks the sun. U.S. Pat. No. 4,583,520 was awarded for this balanced weight az-el tracker on Apr. 22, 1986.

One distinct disadvantage of az-el trackers is that difficulties arise when they are installed in tropical locations, that is, anywhere between the tropic of capricorn and the tropic of cancer; a huge belt around the planet from 23.5 degrees south to 23.5 degrees north. In this region, the sun can and will pass directly overhead, which means that the elevation at that moment will be 90 degrees and the azimuth will be briefly undefined. In this situation the best that an az-el tracker can do is a quick turn-around at noon, since there is a discontinuity of as much as 180 degrees between the data it was following as the sun rose in the east, in comparison to the data it must follow for the descent of the sun in the west. Simplistic tracking schemes could easily be confused to the point of no longer working when confronted with such an extreme discontinuity.

The problem is considerably simplified with a polar-axis tracker. Such a tracker works because in truth the motion of the sun is not at all arbitrary, rather it is very simple and predictable. The apparent motion of the sun is better understood as the earth's motion relative to the sun. The primary apparent motion of the sun is due to a constant rate rotation of the earth about its polar (rotational) axis. In order to accurately account for this motion with a tracker, a stable rotational axis must be created in accurate alignment with the earth's polar axis. Rotating the tracker's rotational axis at a rate which is equal but opposite to the rotational rate of the earth allows anything attached to that axis to stay at a fixed orientation relative to the sun. Solar trackers which take this approach are generally called polar-axis solar trackers, to better distinguish them from the previously mentioned az-el solar trackers.

FIGS. 2 a-2 c show an illustrative sampling of polar-axis solar trackers. The tracker in FIG. 2 a (from U.S. Pat. No. 6,284,968, granted Sep. 4, 2001) has many of the disadvantages of the tracker in FIG. 1 a: the dish is high above the ground, and large torques are required in order to track the sun.

FIG. 2 b (from U.S. Pat. No. 4,368,962, granted Jan. 18, 1983) shows a system of trackers driven by a common set of drive motors. The shared motor concept has the potential to reduce costs, but that potential is not fully realized in this case. The imbalanced weight of the dishes would require large torques in order to track the sun, as mentioned with prior trackers. When the system of imbalanced dishes are connected together, the torques accumulate rather than canceling each other out, so that a drive motor handling four dishes must be at least four times as powerful as the single motor was, and thus significantly more expensive. Another drawback of this design is the cumulative backlash that will be involved in the system of all of bevel gears. At each 90 degree junction between drive shafts, a small amount of play will be introduced between one gear and the next. Since there are four such junctions between the control motors and the dishes, that play will be quadrupled, significantly lowering the precision of the tracking.

Note also that in FIGS. 1 a, 2 a and 2 b, the reflective dishes are all quite small relative to the size of the trackers. Since it is the dishes that are collecting the solar energy, and the trackers are only there to properly aim the dishes, it is desirable to have the largest possible dish size for a given tracker size.

FIG. 2 c shows a tracker that brings together many of the best ideas mentioned above into a single and practical design. It was jointly developed by Dr. Wolfgang Scheffler of Austria and Dr. Deepak Gadhia of India. This picture was taken at an installation Dr. Gadhia constructed near Delhi. The polar axis trackers greatly simplify the daily motion, reducing it to a simple rotation. The weights of the dishes are more or less evenly balanced about the axis of rotation, so that smaller torques could be involved in moving the trackers, although in this case weights have been added to each dish to apply torque and thus keep tension on the control cable. The trackers are interconnected in a simple, accurate and practical manner so that they can be driven by a single drive mechanism, which in this case is a gearmotor from an automotive windshield-wiper mechanism, connected to the dishes through a steel control cable and some pulleys. At this installation fourteen dishes are accurately controlled by a single drive motor.

Note that the elliptical Scheffler reflective dishes in the tracker of FIG. 2 c are very different than the circular reflective dishes in all the preceding images: their outer contour is elliptical rather than circular, and they are designed to bring light to a focus at a point to the side of the dish, so that the light from the sun reflects at a 90 degree angle while simultaneously being concentrated. The focus of this document is to present a new invention designed for circular reflective dishes, which are more commonly used. For the purposes of this document, a circular reflective dish is one which is designed to directly face the sun, and bring light to a focal point that is nearly or directly between the center of said circular reflective dish and the sun. Generally the reflective surface of such a dish will follow a parabolic curve, but small deviations are sometimes encountered. An example of this is the dish shown in FIG. 1 b, in which mirror segments were ground to circular contours to reduce manufacturing costs. Generally such a dish will take a circular shape, but again some variations may be encountered. The dish in FIG. 1 b is again an example, with square mirror segments assembled to roughly approximate a circle, and with a empty slot in the dish which accommodates the supporting means. The dish of the working prototype in FIG. 3 is another example, in which triangular segments come together to approximate a circle, but in truth the shape is a multi-sided polygon with a large number of sides, and with two polygons missing to accommodate the supporting means.

The elliptical Scheffler reflective dishes of FIG. 2 c were also developed by and named for Wolfgang Scheffler. They are valuable for concentrating sunlight on a stationary point throughout the day, despite the motion of the sun. This is particularly useful for solar cooking applications, such as in this case, in which stationary water boilers generate steam for later cooking. Any accurate solar tracker must also account for the seasonal motion of the sun. These trackers do not themselves account for that motion, rather the Scheffler dishes are manually flexed along their long axis so as to attain the tightest possible sun spot on the water boiler, and this accounts for the seasonal motion of the sun. Since the sun moves very slowly through the seasons, this manual adjustment can be done periodically rather than every day. The only disadvantage of this excellent design is that the dishes are highly exposed to winds, especially the dishes which are concentrating light onto the upper side of the boilers. This problem, which is relatively minor near Delhi, becomes progressively worse at higher latitudes, because the entire assembly must be rotated to stay in parallel with the earth's rotational axis, which at higher latitudes will put the upper dish higher and higher above the ground.

It is an objective of this invention to bring together all of the best qualities of the Scheffler/Gadhia solar tracker, and improve on them where possible, for circular reflective dishes rather than for elliptic Scheffler reflective dishes.

It is an objective of this invention to have the weights of the solar tracker balance each other out, minimizing the torques required to maneuver the reflective dish.

It is an objective of this invention to have a single control motor drive multiple reflective dishes, thus reducing the cost of the system.

It is an objective of this invention to minimize the size of the solar tracker relative to the size of the reflective dish, thus collecting the most possible sunlight with a minimum tracker cost.

It is an objective of this invention for the solar tracker to accommodate the seasonal motion of the sun in a manner which can be either manual or automatic, to permit a further cost reduction (by omitting the automatic drive system) in cases where it is possible and appropriate.

It is an objective of this invention to keep the reflective dish as close as possible to the ground, so as to improve the system's ability to withstand wind storms.

It is an objective of this invention to enable the complete inversion of the reflective dish, since that will reduce the dust build-up on the reflective surfaces, and further improve the system's ability to withstand wind storms.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 a and 1 b show two prior-art azimuth-elevation solar trackers.

FIGS. 2 a, 2 b, and 2 c show three prior-art polar axis solar trackers.

FIGS. 3 a and 3 b are photographs of a working prototype of an embodiment of this invention.

FIG. 4 shows a detailed side view of the preferred embodiment of this tracker.

FIGS. 5 a-5 e show the preferred embodiment in various positions and configurations.

FIG. 6 a-6 d show embodiments of fine-tuning adjustment systems, and an automatic declination adjustment means.

FIG. 7 shows embodiments of the drive components configured for driving multiple trackers FIGS. 8 a and 8 b show top views of one embodiment of the tracker support base.

Table 1 shows the declination of the sun for each day of the year.

Table 2 shows the daily changes in the declination of the sun for each day of the year.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIGS. 3 a and 3 b, a working prototype of one embodiment of this invention is shown photographically from two different views. While this prototype has been useful in proving the essential concepts of this invention to be valid and workable, the preferred embodiment is described in the following text and figures, and not in these photographs. The photographs are provided because they help the reader to better visualize the 3-dimensional structure and shape of the preferred embodiment, which may be harder to grasp from the 2-dimensional drawings.

The fundamental details of the preferred embodiment are shown schematically in FIG. 4. Support base 401 provides a stable platform for the tracker, the long axis of which should be accurately aligned in a true north-south direction. FIG. 4 is drawn for a tracker at 28.46 degrees latitude, which would be appropriate for an installation near New Delhi, India. FIG. 4 shows a West-looking view of that tracker, so that rightward on the drawing is North, while leftward is South.

Leveling bolts 402 at each of the four corners help to bring the support base accurately into level. To better withstand high winds, these bolts can be made much longer and anchored firmly in cement. Clamping assembly 403 firmly grasps tracker frame 406 and holds it in place, pushing it against two fixed stops which form the other half of the clamping assembly, and which also hold the two halves of the support base together. These fixed stops are visible in FIGS. 8 a and 8 b.

Tracker frame 406 is a novel and unique aspect of this invention. Because it is in the shape of a 180 degree arc, clamping assemblies 403 (and optionally 405) can hold it firm at virtually any angle. This allows polar axle 407, supported by tracker frame 406, to be held at an angle such that it will be in parallel with the earth's polar axis. FIG. 4 shows the tracker frame tilted at an angle of 28.46 degrees. This angle is measured from the horizontal to match the angle formed by polar axle 407, mounted to tracker frame 406. The correct angle for a given installation site is the same as the geocentric or spherical latitude of that site, which is close to but not exactly the same as the site's geodetic latitude. For sites south of the equator, the tracker would be flipped around, with North on the left and South on the right, so that the lowest side of the axle is on the side of the tracker which is closest to the equator.

For higher latitudes which will require the tracker frame to be further tilted, optional triangular supports 404 hold an additional clamping assembly 405 which provides additional stability to tracker frame 406.

Note that all other known polar-axis trackers require a base or support system which either pivots and lifts the dish high in the air, or else is custom-built according to the latitude of the installation site, which complicates keeping the parts in stock for those trackers. In contrast, tracker frame 406 keeps the dish quite low to the ground, reducing exposure to winds, while simultaneously allowing a single set of tracker parts to work well at almost any installation site on earth.

In order to account for and follow the primary motion of the sun, the polar axle must be turned at a rate of 1 revolution per day by the polar axle rotation means. In this embodiment, the polar axle rotation means includes a gear motor connected through pulleys to polar axle drive pulley 408, which in turn rotates the polar axle. Bearings 409 firmly hold the ends of the polar axle in place and minimize the rotational friction, thus minimizing the torques involved. The dish assembly consisting of reflective dish 412, dish support member 411 and solar energy receiver 413 are very nearly balanced about pivot rod 410 and hence polar axle 407, which also helps to minimize the torques involved in the polar axle rotation. Note that solar energy receiver 413 can take a number of forms, as there are several types of technologies for converting solar energy into other useful forms. If electricity is immediately desired, it could take the form of concentrating photovoltaic (CPV) cells. Or if there are price breakthroughs in heat engines such as Stirling Engines, that could be used to create electricity. Alternatively, the heat could be absorbed with some kind of thermal transfer fluid, and transported and/or stored for later use, or later conversion into electric energy.

Besides the apparent daily motion of the sun, there is also a seasonal motion. In astronomical terms, the declination of the sun describes the apparent north-south motion of the sun as seen from the earth. A declination angle of zero means that the sun is in alignment with the equator, which occurs at particular times on March 21 st and September 23rd. The declination of the sun peaks on about June 22nd at an angle of 23.43 degrees north of the equator, and reaches its minimum on about December 22nd at an angle of 23.43 degrees south of the equator.

Within this document, the term declination angle is used not only for the declination of the sun above or below the equator, but also for the angle formed by the dish assembly of this tracker, which mimics that celestial angle. In FIG. 4, the dish assembly is shown at an angle perpindicular to the polar axle, thus perpindicular to the earth's rotational polar axis, which aligns it with the earth's equator. This angle of the dish assembly therefore corresponds to a declination angle of zero, which would align the tracker with the sun's declination on March 21st and September 23rd. A declination adjustment means is used to adjust the tracker's declination angle, bringing the tracker components into alignment with the current declination angle of the sun. In this embodiment, the declination adjustment means takes the form of adjustable length turnbuckle 415, which is shortened or lengthened so as to pivot the dish assembly to the correct angle. Pivot rod 410 defines the declination adjustment axis, which can be envisioned as coming out of the paper at the center of pivot rod 410, about which the dish assembly is pivoted. Northern declination angles would be achieved by lengthening adjustable length turnbuckle 415, so as to pivot the dish assembly clockwise. See also FIG. 5 b. Southern declination angles would be achieved by shortening turnbuckle 415 so as to pivot the dish assembly counter-clockwise. See also FIG. 5 a. There is very little torque involved in either pivoting motion, since the components of the dish assembly are very nearly in balance about pivot rod 410 as previously mentioned.

Note that in this embodiment, all components of the dish assembly are constructed such that they cannot collide with tracker frame 406 at any angle of motion about polar axle 407, nor at any declination angle between plus or minus 23.43 degrees. Circular reflective dish 412 is also illustrated in a side view, 414, to better illustrate the split-dish construction, and the all-around clearance that is another result of having the tracker frame shaped as a circular arc. The split in the dish is required in order to allow room for the polar axle as the dish pivots for variations in the declination angle.

Before moving on to other figures, note that FIG. 4 illustrates this embodiment of the tracker as it would appear at local solar noon on about March 21st or September 23rd when installed at a site near New Delhi. Local solar noon is an astronomical term meaning the moment in the day when the sun is at its highest point at a particular site, which generally happens close to noon in any time zone, but is unaffected by legal definitions such as daylight savings time or time zone boundaries. The latitude of the site is determined, as mentioned before, by the angle of polar axle 407 relative to the horizontal, which in this case is about 28.5 degrees. The day of the year is determined by the declination angle, which is the angle of the dish assembly relative to the equator, or in other words relative to a line perpindicular to polar axle 407, as is better illustrated in FIGS. 5 a and 5 b. The hour of the day is determined by the angle of the polar axle, which in its rotation mimics the rotation of the earth.

In FIGS. 5 a and 5 b this embodiment of the tracker is illustrated at the two seasonal extremes of motion. FIG. 5 a shows how it would appear at local solar noon at winter solstice, while FIG. 5 b shows it at the same hour of summer solstice.

FIG. 5 c illustrates this embodiment in an inverted storage position for the dish. Since dust settles downward 24 hours a day, some of that dust can be kept off the reflective side of the dish by inverting the dish during the night when it's not in use. In this way the required frequency of washings can be reduced for a given level of cleanliness. Another advantage of being able to invert the optics is to better protect them from damage caused by wind or hail storms. An incidental advantage has to do with the economics of scale, in that larger dishes are sometimes economically advantageous. If the reflective dish is to be used with a Stirling Engine, for example, then that dish must be sized to match the capacity of that engine. A 10 kw Stirling Engine will cost less than twice as much as a 5 kw Stirling Engine, so there is an economic advantage of using the largest possible engine, and hence the largest possible dish. The primary factor that limits the size of reflective dishes is their ability to withstand high winds, so being able to invert the dish incidentally allows larger dishes, which allows larger Stirling Engines, which can be economically advantageous.

FIG. 5 d illustrates an embodiment of the tracker base without the triangular supports for near-equatorial latitudes. In this case there is no need for them, as all portions of the tracker are quite near the ground.

FIG. 5 e illustrates an embodiment of the tracker assembled with the polar axle drive pulley on the top side, for extreme latitudes in which there's insufficient space to accommodate that on the lower side. This is a case in which the triangular supports play a much more important role in supporting the tracker against lateral winds. Here the tracker is configured for a latitude of 58.5°, which would correspond to parts of Canada, Alaska, Sweden and Russia.

Note that all of the other illustrations (excepting 5 d and 5 e) show an embodiment of the tracker configured for a latitude of 28.5°, corresponding to parts of India, China, Northern Africa, Mexico, Australia and many other locations. All of the configurations shown assume the same basic set of parts.

FIGS. 6 a through 6 d are primarily related to fine-tuning adjustments, which serve to improve the tracking accuracy. In the realm of solar energy it is extremely important to minimize the costs involved. Often the precision involved in making a given part will play a substantial role in the cost of that part, so it is helpful if the highest possible precision can somehow be attained from a system that is built with components of only moderate (rather than high) precision. All of the following fine-tuning adjustments have the purpose of getting an embodiment of the tracker to track the sun as accurately as possible.

FIG. 6 a shows the details of one embodiment of a polar axle rotational fine-tuning means. The assembly shown would alternately take the place of polar axle drive pulley 408, serving the same function, but adding the ability to fine tune the rotational position of the tracker. When multiple trackers are all driven by the same polar axle rotation means, that rotation means will at best be able to consider the position of one solar tracker, and keep it in good alignment with the sun. In order to maintain a comparable level of accuracy in all the other trackers being driven, it is essential that each of those trackers be aligned as closely as possible with the first tracker. One part of that alignment is served by having a good common reference, which is the sun, but a practical means must also be available to measure any misalignment with the sun. That means of measurement is the shadow cast by the solar energy receiver on the dish-mounting components at the center of the reflective dish. Using that as a reference, any misalignment angle with the sun is easy to see.

The second part of correcting that misalignment is having an easy method of making a fine-tuning adjustment, which is the purpose of the polar axle rotational fine-tuning means, one embodiment of which is shown in FIG. 6 a. The left side of 6 a is a front view of those components, with a side view on the right. Hub piece 601 is mounted onto the end of the polar axle in such a way that it is firmly attached and completely prevented from rotating relative to that axle, for example with a keyed shaft and a tapered key. Hub piece 602 is bolted to Hub piece 601 in such a way that the two together clamp down on outer ring 603 when bolts 604 are tightened. When the bolts are loosened, however, the two hub pieces 601 and 602 can be rotated relative to outer ring 603. In order to make this adjustment with more precision, a long steel bar can be placed along the hub, below the level of the bolt heads, and then rotated slowly in the direction needed, while holding the outer ring fixed in place. The steel bar would thereby apply torque to the bolt heads, which would turn the hub pieces relative to the outer ring in a much more controlled and accurate manner. Although it is not again mentioned below, simple tools such as just mentioned can be used to similarly make adjustments on many of the following fine-tuning methods, with considerably greater precision than without such tools. Since the cost for such tools does not add in any way to the cost of the tracker, this is an effective method of increasing tracker accuracy without contributing to its price.

In FIG. 6 b the details are shown for one embodiment of a fine-tuning system for the North-South direction of the polar axle. Two views are shown; the top view shows details of bearing pillow block 610, while the bottom view shows that same pillow block in place, assembled with all the components around it. Pillow block 610 supports one end of the polar axis, which is not visible in this view, but which is 620 in FIG. 6 d on the same page. By making small left-right adjustments to the position of pillow block 610, the angle of the polar axis relative to the north-south direction is thereby adjusted by very small angles. Extra-length slots 613 are provided in pillow block 610 for this purpose. Bolts 612 firmly hold pillow block 610 in position, clamping it to cross-beam 611. These bolts can be loosened for the purpose of making this adjustment.

FIG. 6 c shows the details of one embodiment of a fine-tuning system for the vertical slant of the polar axle, which fine-tunes the selected latitude. FIG. 6 c is an enlarged section of FIG. 6 d. Bearing pillow block 621 supports the other end of polar axle 620, and is held in place by bolts 622. Firm springs 623 are introduced between pillow block 621 and mounting cross-piece 624. Mounting cross-piece 624 is welded to tracker frame 625, which holds the whole assembly firmly in place. By tightening bolts 622, firm springs 623 are further compressed and pillow block 621 is moved slightly downward, lowering this end of polar axle 620. By loosening bolts 622, the process is reversed: firm springs 623 extend, pillow block 621 is moved slightly upward, and this end of polar axle 620 is vertically raised. Raising and lowering this end of polar axle 620 slightly changes the vertical slant of the polar axle.

Another fine-tuning adjustment is shown in FIG. 6 d. Leveling bolts 627 allow tracker support base 626 to be brought to level.

If an automated declination adjustment is desired, one embodiment of such an adjustment means is also shown in FIG. 6 d. Some of these components replace other manual components which have been previously discussed. Instead of adjustable-length turnbuckle 415, threaded rod 635 can be used for the same purpose, which would be driven by motor 637. Pivoting mounts 636 would hold the motor in place at one end of the rod, as well as holding swiveling nut 638 in place on the rod. When motor 637 is activated to turn threaded rod 635, swiveling nut 638 is brought closer or further according to the direction the rod is being turned. This will bring dish support member 631 closer or further to polar axle 620, and hence pivot the dish assembly about pivot rod 630, changing the tracker declination.

FIG. 3 a already showed a photograph of multiple dishes being driven by a single drive motor, while FIG. 3 b showed a photographic view of one embodiment of the drive elements involved. FIG. 7 schematically illustrates a similar embodiment of the same drive components. Drive pulley 701 is attached to a gearmotor, not shown, driven by a control circuit. Pulley assemblies 702 guide cable 703 in the path shown around a plurality of polar axle drive pulleys 708, which rotate the tracker axles (not visible here) which are supported by bearings 709. Bearings 709 are bolted to cross beams 705, which are in turn welded to tracker frames 706. (Components 706-709 correspond to components 406-409 in FIG. 4). Cross beams 705 further support pulley assemblies 702, with a slightly lengthened cross-beam also supporting drive pulley 701 and the gear motor attached to it. Tension is maintained on cable 703 by means of tensioning springs 704, and this tension can be adjusted by tension-adjusting turnbuckle 710.

In this illustration embodiments of three solar trackers are shown, each of which has been configured slightly differently as the system of trackers was installed. The leftmost tracker is configured to be at the end of a string of trackers, thus there's only need for one pulley assembly 702. The middle tracker is configured to be in the middle of the string, with two pulley assemblies 702. For a string of 12 trackers, there would be 10 middle trackers configured like this one. The tracker on the right, finally, is configured to be driven directly from the drive motor, and all the associated drive circuitry (including sensors indicating the position of the trackers relative to the sun's position) would be installed on this tracker. The cables would be connected together as shown, with tensioning springs between them, in order to get all of the trackers in a string to track the sun in unison.

The configuration of optional tensioning springs shown here has two purposes. First, in locations where the system of trackers will undergo large deviations in temperature, the steel cables will alternately undergo thermal expansion and contraction. In the configuration shown, the thermal length shifting in the steel cables is balanced by compensating shifts in the lengths of the tensioning springs which are distributed throughout the length of the cable. This serves to minimize the net angular shifting of any tracker, insuring maximum tracking accuracy in every tracker. Second, in locations with high winds, a sudden gust of wind could act on one dish, or a few dishes, or all of the dishes nearly simultaneously. With the configuration of tensioning springs shown, some of the energy of such gusts is harmlessly absorbed and dissipated by the springs, which would then quickly bring the dishes back to their intended orientations. This energy might otherwise be absorbed by the reflective dish, causing distortions or greater damage. Thus the springs can help to minimize wind damage to the system, as well as reducing the possibility that such gusts would disorient any of the dishes due to cable slippage.

The friction between the steel drive cable 703 and the pulleys it interfaces with (701 & 708) must be sufficiently high to prevent slippage, so the tensioning springs must be adequately stretched via turnbuckle 710 to insure this, and a material with a high coefficient of friction should be used to cover the pulley surfaces. Exorbitant tension is not needed, since the design shown includes large angles of working contact on the drive pulleys, and friction increases exponentially with the coefficient of friction between the two materials and the angle through which there is working contact, in radians.

In FIGS. 8 a and 8 b, a top view of one embodiment of the support base is shown, with and without the tracker frame in place. As described previously in FIG. 4, support base 801 provides a stable platform for the tracker, the long axis of which should be accurately aligned in a true north-south direction. Leveling bolts 802 at each of the four corners of the base are provided to help bring the support base accurately into level. The base firmly holds tracker frame 806 in place with top clamp 803 acting against fixed stops 807. This is the same action as was described earlier with clamping assembly 403 in FIG. 4, but in this view additional details are visible. Fixed stops 807 also act to hold the two halves of the support base together. For higher latitudes which will require a greater angle of tilt, triangular supports 804 hold an additional clamping assembly 805 which also provides stability to C-piece 806.

While the tracker described herein is intended primarily for countries in which labor is inexpensive and materials are expensive, it can be readily adapted for other countries, with the simple addition of an automatic declination adjustment, as in FIG. 6 d. However, even in well-developed countries it would be worth evaluating whether this is really needed. The fastest that the sun ever changes its declination is at a rate of only 0.4 degrees per day (in March and September, when the sun is near the equinoxes), and there are periods when the declination changes much slower than this. Since the declination is changing over a 24 hour day and the sun is only up for about 12 of those hours, the declination would change at most 0.2 degrees during those daylight hours. If that change were accurately anticipated, by knowing the data shown in table 2, then the tracker declination could be manually set each morning to be at the midpoint of the solar declination throughout that day, theoretically attaining 0.1 degree accuracy, or better, through at least a majority of the day. This is for a worst-case day; on the best case days the declination does not change at all! Adjusting the trackers by hand is a very fast process, taking as little as 10 seconds per tracker, so very little labor is involved. Ultimately the choice will depend on the economics of the situation, the economic conditions where the trackers are to be installed, and the accuracy requirements of the solar energy receiver and the circular reflective dish.

To better understand and weigh this trade-off, tables 1 and 2 are included. Table 1 shows the declination angle of the sun for every day of the year, based on the data published online at: www.wsanford.com/˜wsanford/exo/sundials/DEC_Sun.html. Note that positive numbers indicate that the sun is above the Northern hemisphere, while negative numbers indicate that the sun is above the Southern hemisphere. Table 2 shows how the solar declination angle changes on each day of the year; it is based the data in Table 1.

While one embodiment of this invention with several options has been presented above, many changes can be made without departing from the spirit and scope of the invention. For example, there is no need for the support base to be flat or level, rather it might make sense to incorporate elements of a support base into a new structure which is already under construction for different purposes, but which would be well served by having solar energy collectors mounted on it. Any suitable solar energy receiver may be utilized with this invention, including Stirling engines, concentrating photovoltaic cells, solar-thermal collectors, or others as may be introduced in the future. The shape and size of the dish support member would naturally change so as to better accommodate the needs of those solar energy receivers. The various insights embodied in this invention enable the production of solar trackers for circular reflective dishes at significantly reduced costs, while still attaining tracking accuracies within small fractions of a degree. The scope of this invention should be determined by the appended claims and their legal equivalents, rather than by the explanations or illustrations here presented.

TABLE 1 Solar declination angles for each day of the year JAN- FEB- SEP- OC- NO- DE- Day UARY RUARY MARCH APRIL MAY JUNE JULY AUGUST TEMBER TOBER VEMBER CEMBER 1 −23.07 −17.33 −7.82 4.30 14.90 21.97 23.15 18.17 8.50 −2.95 −14.23 −21.72 2 −22.98 −17.05 −7.43 4.70 15.20 22.10 23.08 17.92 8.15 −3.33 −14.57 −21.87 3 −22.90 −16.77 −7.05 5.08 15.50 22.23 23.02 17.67 7.78 −3.73 −14.88 −22.02 4 −22.80 −16.47 −6.67 5.47 15.78 22.37 22.93 17.40 7.42 −4.12 −15.18 −22.17 5 −22.70 −16.17 −6.28 5.85 16.08 22.48 22.85 17.13 7.05 −4.50 −15.50 −22.30 6 −22.60 −15.87 −5.90 6.22 16.37 22.58 22.75 16.87 6.67 −4.88 −15.80 −22.42 7 −22.47 −15.57 −5.50 6.60 16.65 22.70 22.65 16.60 6.30 −5.27 −16.10 −22.53 8 −22.35 −15.25 −5.12 6.98 16.92 22.78 22.55 16.32 5.93 −5.65 −16.40 −22.65 9 −22.22 −14.93 −4.73 7.35 17.20 22.88 22.43 16.03 5.55 −6.03 −16.68 −22.77 10 −22.08 −14.62 −4.33 7.72 17.45 22.97 22.32 15.75 5.17 −6.42 −16.97 −22.87 11 −21.93 −14.30 −3.95 8.12 17.72 23.03 22.18 15.45 4.80 −6.80 −17.25 −22.95 12 −21.78 −13.97 −3.55 8.47 17.98 23.12 22.07 15.17 4.42 −7.17 −17.53 −23.03 13 −21.62 −13.63 −3.17 8.83 18.23 23.18 21.92 14.87 4.03 −7.53 −17.80 −23.12 14 −21.45 −13.30 −2.77 9.18 18.48 23.23 21.77 14.55 3.65 −7.92 −18.07 −23.18 15 −21.27 −12.97 −2.37 9.55 18.72 23.28 21.62 14.25 3.27 −8.30 −18.33 −23.23 16 −21.10 −12.62 −1.98 9.90 18.97 23.33 21.47 13.93 2.88 −8.67 −18.58 −23.28 17 −20.90 −12.27 −1.58 10.27 19.18 23.37 21.30 13.62 2.50 −9.03 −18.83 −23.33 18 −20.70 −11.92 −1.18 10.62 19.42 23.40 21.13 13.30 2.10 −9.40 −19.08 −23.37 19 −20.50 −11.57 −0.80 10.97 19.63 23.42 20.97 12.98 1.72 −9.75 −19.32 −23.40 20 −20.30 −11.22 −0.40 11.32 19.85 23.43 20.78 12.65 1.33 −10.12 −19.55 −23.42 21 −20.08 −10.87 0.00 11.65 20.07 23.43 20.60 12.32 0.95 −10.48 −19.78 −23.43 22 −19.87 −10.50 0.40 12.00 20.27 23.43 20.40 11.98 0.55 −10.83 −20.00 −23.43 23 −19.63 −10.13 0.78 12.33 20.47 23.43 20.20 11.65 0.17 −11.20 −20.22 −23.43 24 −19.40 −9.77 1.18 12.67 20.65 23.42 20.00 11.32 −0.23 −11.55 −20.43 −23.43 25 −19.17 −9.40 1.58 13.00 20.83 23.40 19.78 10.97 −0.62 −11.90 −20.63 −23.42 26 −18.92 −9.03 1.97 13.32 21.02 23.38 19.57 10.63 −1.00 −12.23 −20.83 −23.38 27 −18.67 −8.65 2.37 13.63 21.20 23.35 19.35 10.28 −1.40 −12.58 −21.02 −23.35 28 −18.42 −8.28 2.75 13.97 21.37 23.32 19.13 9.93 −1.78 −12.92 −21.20 −23.32 29 −18.15 −8.05 3.15 14.27 21.52 23.27 18.90 9.58 −2.17 −13.25 −21.38 −23.27 30 −17.88 3.53 14.58 21.68 23.22 18.67 9.22 −2.57 −13.58 −21.55 −23.20 31 −17.62 3.92 21.83 18.42 8.87 −13.92 −23.13

TABLE 2 Changes in the Solar declination angle for each day of the year JAN- FEB- SEP- OC- NO- DE- Day UARY RUARY MARCH APRIL MAY JUNE JULY AUGUST TEMBER TOBER VEMBER CEMBER 1 0.07 0.28 0.23 0.38 0.32 0.13 −0.07 −0.25 −0.37 −0.38 −0.32 −0.17 2 0.08 0.28 0.38 0.40 0.30 0.13 −0.07 −0.25 −0.35 −0.38 −0.33 −0.15 3 0.08 0.28 0.38 0.38 0.30 0.13 −0.07 −0.25 −0.37 −0.40 −0.32 −0.15 4 0.10 0.30 0.38 0.38 0.28 0.13 −0.08 −0.27 −0.37 −0.38 −0.30 −0.15 5 0.10 0.30 0.38 0.38 0.30 0.12 −0.08 −0.27 −0.37 −0.38 −0.32 −0.13 6 0.10 0.30 0.38 0.37 0.28 0.10 −0.10 −0.27 −0.38 −0.38 −0.30 −0.12 7 0.13 0.30 0.40 0.38 0.28 0.12 −0.10 −0.27 −0.37 −0.38 −0.30 −0.12 8 0.12 0.32 0.38 0.38 0.27 0.08 −0.10 −0.28 −0.37 −0.38 −0.30 −0.12 9 0.13 0.32 0.38 0.37 0.28 0.10 −0.12 −0.28 −0.38 −0.38 −0.28 −0.12 10 0.13 0.32 0.40 0.37 0.25 0.08 −0.12 −0.28 −0.38 −0.38 −0.28 −0.10 11 0.15 0.32 0.38 0.40 0.27 0.07 −0.13 −0.30 −0.37 −0.38 −0.28 −0.08 12 0.15 0.33 0.40 0.35 0.27 0.08 −0.12 −0.28 −0.38 −0.37 −0.28 −0.08 13 0.17 0.33 0.38 0.37 0.25 0.07 −0.15 −0.30 −0.38 −0.37 −0.27 −0.08 14 0.17 0.33 0.40 0.35 0.25 0.05 −0.15 −0.32 −0.38 −0.38 −0.27 −0.07 15 0.18 0.33 0.40 0.37 0.23 0.05 −0.15 −0.30 −0.38 −0.38 −0.27 −0.05 16 0.17 0.35 0.38 0.35 0.25 0.05 −0.15 −0.32 −0.38 −0.37 −0.25 −0.05 17 0.20 0.35 0.40 0.37 0.22 0.03 −0.17 −0.32 −0.38 −0.37 −0.25 −0.05 18 0.20 0.35 0.40 0.35 0.23 0.03 −0.17 −0.32 −0.40 −0.37 −0.25 −0.03 19 0.20 0.35 0.38 0.35 0.22 0.02 −0.17 −0.32 −0.38 −0.35 −0.23 −0.03 20 0.20 0.35 0.40 0.35 0.22 0.02 −0.18 −0.33 −0.38 −0.37 −0.23 −0.02 21 0.22 0.35 0.40 0.33 0.22 0.00 −0.18 −0.33 −0.38 −0.37 −0.23 −0.02 22 0.22 0.37 0.40 0.35 0.20 0.00 −0.20 −0.33 −0.40 −0.35 −0.22 0.00 23 0.23 0.37 0.38 0.33 0.20 0.00 −0.20 −0.33 −0.38 −0.37 −0.22 0.00 24 0.23 0.37 0.40 0.33 0.18 −0.02 −0.20 −0.33 −0.40 −0.35 −0.22 0.00 25 0.23 0.37 0.40 0.33 0.18 −0.02 −0.22 −0.35 −0.38 −0.35 −0.20 0.02 26 0.25 0.37 0.38 0.32 0.18 −0.02 −0.22 −0.33 −0.38 −0.33 −0.20 0.03 27 0.25 0.38 0.40 0.32 0.18 −0.03 −0.22 −0.35 −0.40 −0.35 −0.18 0.03 28 0.25 0.37 0.38 0.33 0.17 −0.03 −0.22 −0.35 −0.38 −0.33 −0.18 0.03 29 0.27 0.23 0.40 0.30 0.15 −0.05 −0.23 −0.35 −0.38 −0.33 −0.18 0.05 30 0.27 0.38 0.32 0.17 −0.05 −0.23 −0.37 −0.40 −0.33 −0.17 0.07 31 0.27 0.38 0.15 −0.25 −0.35 −0.33 0.07 

1. A polar-axis solar tracker comprising: a) A strong and stable support base capable of aligning a tracker frame in a true North-South direction, and firmly holding it in that alignment. b) A tracker frame in the shape of a letter “C”; substantially like a 180 degree arc, which owing to its shape can be mounted to the support base at any angle, particularly such that the endpoints of the arc are inclined from the horizontal at an angle equal to the geocentric latitude of the installation site, so that said endpoints are in parallel alignment with the earth's polar (rotational) axis. c) A polar axle rotatably attached to said endpoints of said tracker frame, which is thereby positioned in parallel alignment with the polar axis of the earth, so that as the earth rotates the polar axle can be rotated in the opposite direction, and thus hold all things mounted on the polar axle in a substantially fixed orientation relative to the sun, despite the rotation of the earth. d) A dish support member pivotally attached to the center of the polar axle so that it can pivot through declination angles of plus or minus 23.43 degrees (or more) towards said endpoints of the tracker frame, so that a circular reflective dish mounted on said dish support member can follow the sun's seasonal motion above and below the earth's equator. e) A circular reflective dish supported on one end of said dish support member. f) A solar energy receiver supported at the other end of said dish support member, positioned substantially at the focal point of said circular reflective dish. g) A polar axle rotation means, capable of rotating the polar axle and all components attached to it at a rate of one revolution per day. h) A declination adjustment means, capable of adjusting the pivot angle of the dish support member and all other components attached to it, so as to follow the seasonal motion of the sun.
 2. A polar-axis solar tracker, as defined in claim 1, in which said circular reflective dish is slotted or split so as to allow passage and clearance for said polar axle when said dish support member is pivoted through its full range of motion.
 3. A polar-axis solar tracker, as defined in claim 1, in which said circular reflective dish is sized and shaped so as not to collide with said tracker frame at any angle of the dish support member pivoting motion, nor any angle of the polar axle rotation.
 4. A polar-axis solar tracker, as defined in claim 1, in which said solar energy receiver is sized, shaped and positioned so as not to collide with said tracker frame at any angle of the dish support member pivoting motion, nor any angle of the polar axle rotation.
 5. A polar-axis solar tracker, as defined in claim 1, in which said solar energy receiver is positioned and/or weighted such that the pivot point of said dish support member is at or very near the center of mass of the dish assembly, comprising the solar energy receiver, the dish support member and the circular reflective dish, thus making it possible to pivot the dish assembly and rotate the polar axle with a minimum of torque.
 6. A polar-axis solar tracker, as defined in claim 1, in which said polar axle rotation means is capable of simultaneously rotating the polar axles of multiple solar trackers at the same time, thus eliminating the need for multiple polar axle rotation means, and thus significantly reducing the cost when installing multiple trackers.
 7. A polar-axis solar tracker, as defined in claim 1, in which said polar axle rotation means is a system of one or more cables and pulleys, and hence induces zero backlash, as is commonly caused by gears.
 8. A polar-axis solar tracker, as defined in claim 1, in which said polar axle rotation means is a system of cables and pulleys, with tensioning springs positioned periodically between said cables in such a way that the thermal expansion or contraction of said cables is absorbed by the springs, and causes minimal or zero misalignment in the driven polar axles.
 9. A polar-axis solar tracker, as defined in claim 1, with provisions for the following fine-tuning adjustments: a) fine-tuning adjustment(s) for bringing the support base precisely into level. b) Horizontal fine-tuning adjustment(s) for bringing the polar axle into alignment with the North-South direction at the installation site. c) Vertical fine-tuning adjustment(s) for bringing the inclination or slant of the polar axle into alignment with that of the earth's polar (rotational) axis. d) Rotational fine-tuning mean adjustment(s) to fine-tune the rotational angle of the polar axle relative to that prescribed by said polar axle rotation means, so that when multiple trackers are driven by the same rotation means, each of them can be brought independently into alignment with the direction of the sun.
 10. A polar-axis solar tracker, as defined in claim 1, in which said declination adjustment means can optionally be a mechanism which is periodically and manually re-aligned with the sun, thus avoiding the costs of an automated adjustment means, and further reducing the overall costs of the system.
 11. A polar-axis solar tracker, as defined in claim 1, in which said polar axle rotation means can be driven so as to completely invert said dish assembly at night, or at other selected occasions, for reduced accumulation of dust on said circular reflective dish, or to better withstand high winds or other potentially harmful situations. 